Continuous distillation of coal tar



Jan. 13, 1959 H. R41-TE E'rAl., 2,858,713

coNTIINUous nIsTILLATIoN 0F com. TAR

Filed Aug. 2, 1954 k FIRST l MAIN coLuMN wvelvroks: HEM/mcy RATTE JUL/U5GELLER United, States Patent VC) CONTINUOUS DISTILLATION OF COAL TAR fHeinrich Ratte, Frankfurt arn'Main, and Julius-Geller, BadA Homburg vor`der Hohe, Germany, assignors to RtgersWerke'-Aktiengesellschaft,Frankfurt am Main, Germany Application August 2, -1954,Serial No.447,182I

Claims `priority,lapplication Germany'May `14, 1949 4 Claims.v (Cl.208-42) For separating coal tar by continuous distillation into severalfractions various methods have hitherto been employed.v It has beenknown to separate coal tar by distillation-in'successive stages, thewater and the light oil vapors first formed being distilled off underatmospheric pressure and condensed in a cooler, while lthe constituentsAof .highery .boiling points evaporated in' ,the next .following stepsare drawn .off under vacuum. and treated in fractionating columns. In.this procedure only a few fractions with relatively wide .boilingrangesare obtained.. Moreover, the Vheatconsumption is very vconsiderable, as.in .this lprocedure recovery of heat from the fractions .is possibleonly to a limited extent. Finally, in this known method the tar issuperheated, so thatrthe yields ofthe oil fractions to be recovereddecrease to ka correspondi-ng extent.

In. another .known process the crudeV coal tar isltrst continuouslyheated and dehydrated in a tubev furnace and then y'heated in a .secondtube furnace until all .volatile constituents `with the exception of thepitch-like residue are evaporated. The vapors thus formed are-subjectedto'fractional condensation inv one large column orrin several .columnsarranged onebehindy the other. In the latter case, pitch will accumulatein the sump vof the first column,';while at thel head, through partialcondensation; high boiling oils, e. g. anthracene oils, are recovered inthe treatment,y of coke-.oven tar;k The constituents .not condensed insaid first column are conveyed to the second column, iinithe sumpv .ofwhich oils of lower boiling point are collected,.1andso:on, while thevaporsleavingfthehead of the. last column .consistxof light oils.

In .order to -avoid .undue overheating of the charge, n this lastmentioned process, it has alreadyzbeen :proposed to operate* thefractionatingcolumns undenvacuum. However, Aas' the.. entire .chargeincluding `the low-'boiling constituents .must be, heated/in :thetubefurnaces to .the required 'maximum 'r distillation temperature', anotxinf considerable. part Vof the volatile constituents will vbe sub-ijected to unnecessary. `thermal stress, .so -that thefoil yield. is:lcorrespondingly reduced; Furthermore: .then-.partial superheating of thecharge results in anincreasednheat consumption. The fact that thelow-boiling constituents accompany the higher boiling ingredients on'-their' way through the column and are thelast to be condensed, rendersfractionationlmore ditiicult.I Furthermore,fo`r the vsame reason, :thecondensation heat of .thevapors cannot Vbe sufficiently utilized forpreheating 4the crude. tar.y

The kknown process has the further;V disadvantage,V that the-columns, inwhich `the low-boiling fractionsare ,rer

covered, are under the :highest vacuum, while. qthe. preceding columns,in .which fractions .having higher b oil. ing points .are treated,yoperate -undena lowerfvacuum,l corresponding to the pressu-re .losswithin the-columns.- Consequently, just in .thefractionation ofhigher-boiling constituents, where -a specially: lowpressure lisdesirable,- the..distillation temperatures. .cannot Abe .reduced-:to.the full desired extent. Moreover, the great quantities of w 2,868,713Patented Jan.` 13, 1959 vapors t'o be handled render the production of ahigh vacuum ditlcult. y y

Finally, during heating of the crude tar in the tube furnace to hightemperature corrosions will occur and deposits will .be formed, whichwill result in the necessity of interruptions of the process foroverhauling the tube furnace after it has been in operation for acertain time.

The disadvantages of the known methods are avoided in the vmethodaccording to the present invention, which in addition, makes possible a'particularly economical separation of coal'tar into .a relatively greatlnumber of rather sharply cut individual fractions. The new method isvcarried out in. lseveral fractionating columns connected with each otherin series, whereby the temperatures are increased from column to columnor step to step, while the pressure is decreased from column to column,to a high vacuum of about 10-'100 mm. Hg in the last column. For furtherfractionation of material branched off from main columns, the latter areprovided with one or more side columns. The vacuum in the individualsteps can be adapted or adjusted within wide limits, and thedisplacement of boiling points with respect to one another in'vacuum,can be extensively utilized. The coal tar can be preheated and is thenfed to a middle section of the rst main. column (I) at the head Of whichthe lowest boiling fraction -is taken off. This column isoperated'undera pressure .of about 760 to'llOO mm. Hg. The temperatureat the head of this column is about to 1409 C..and the temperature inthe sump of this column is :kept'by regulation of the reflux and by areheater at about 280 to 320 C. Infone or more side columns of the firstmain column, azcorrespondingnumber of fractions can `be recovered. Theresidue from the sump of the firstcolumnisintroduced into a middlesection of the second main column (Il). Here again the lowest boilingfraction is discharged at the column head. The procedure in the third orany further main column will be similar. The second column and thosefollowing it are kept undera vacuum which isy increased from column tocolumnvor .step by step depending on the nature of the desiredfractions. In order to increase yields as much as' possible and/or tokeep decomposition of pitch-like residuek at a minimum, the lower partof the last main column can be operated under particularly high vacuum.Incontrast tothe knownmethods, in the method according to the presentinvention, a very good vacuum can be easily produced even with highboiling fractions, so that the fractional-distillation proceeds verygently and produces ahigh yield of oil with a relatively low amount ofpitchl residue. As the amounts of vapor to be drawn off are relativelylow, the vacuum can be produced at relatively low cost.

In .the procedure according to the presentinvention the condensationheat of all fractions, including high-boiling amount of heat to the tarand this great amount of heatV is applied to the tar and its fractionsaccording to the invention .in such manner that sharply cut fractionsare obtained xwith. a minimum of decomposition and maximum'savings ofheat. According tothe invention, the

distillation heat is `supplied to the main columns substan`r tiallyvbyrindirect'heating with circulation and l.partial evaporation oftheresidues accumulating Yin the column sump. In order, to avoidoVer-heatingof the pitch-like.

residue lof the highest boiling point, according to the ini vention theresidue of he penultimate main columnwhich .is to be introduced into thelast mainl column, is passed,

before entering the latter, through a heater, Where it isk heated toprovide it with the necessary amount of heat.

In order to carry out heating of the residue accumulating in the columnsump as gently as possible, said residue is circulated through a tubefurnace. In thisway the temperature drop in the heat transfer in thetube furnace `can be reduced and harmful vsuper-heating of the taravoided. As in separating the tar into several fractions the maincolumns are preferably provided with side columns, heat must be suppliedalso to these side columns.` It has been found to be of advantage toheat the sump of these side columns by a portion of distillation residuereheated in the tube furnace. Por this purpose the side columns areprovided with a suitable reboiler or reheater, through which thebranched-off portion of the distillation residue is passed as theheating medium.

An apparatus for carrying out the invention is illustrated by way ofexample in theaccompanying drawing which is a vertical sectionaldiagrammatical view of an apparatus for carrying out the invention.

It is intended to separate coal tar a by distillation into thefractions: light oil b, water c, carbolic oil d, naphtha lene oil I e,naphthalene oil II f, Washing oil g, anthracene oil I h, anthracene oilII i, heavy oils j and pitch k.

The crude coal tar a passes rst into the heat exchangers 1, 2 and 3,where the head vapors of the column, with or without those of the sidecolumns, give olf their condensation heat to the crude c'oal tar. Fromthe heat exchanger 3 the crude tar passes to the heat exchanger 4 forthe intermediate reux of main column III and finally to the heatexchanger 5, where thepitch gives off a portion of its sensible heat.Finally, the extensively preheated crude coal tar is introduced into amiddle section of column I. From the head of this column, the water andthe lowest boiling tar fraction b are taken off and passed through heatexchanger 1 and cooler 6 to the separating vessel 7. The light oil bpasses into the receiver 8, from where the pump 9 conveys a portion ofthe light oil as reux to the head of column I. The quantity of thereturn flow is so adjusted that the temperature in the column headamounts to about 80 to 140o C. From an upper section of the column I aportion is branched otf and introduced into the sidev column 10 in whichit is rectified to separate it from the lower boiling consttutents. Theresidue accumulating in the sump of the column I is circulated by thepump 11 through the tube furnace 12 and kept continuously incirculation. It is returned from the tube furnace into the column underexpansion and partial evaporation and the evaporated constituents risein the column.

A portion'of the residue in the sump of column I, corresponding to thedifference in the inflow of coal tar and the fractions b, c, d of columnI is conveyed through pipe 13 to a middle section of the column Il. Asin the 4column I an absolute pressure of about 760 to 1100 mm. Hg and inthe column II a pressure of about 40 to 400 mm. Hg is maintained, thispressure drop is sufficient for passing liquid from the sump of column Iinto ycolumn II.

From the head of column II the fraction e is taken off, which passesthrought heat exchanger 2 and cooler 14 into receiver 1S. This receiveris connected to vacuum pump 16. A portion of the fraction e is pumped bypump 17 as reflux to the head of the column II, and is adjusted in suchmanner that a temperature of about 120 to 190 C. is maintained there.

The higher boiling fractions and g are drawn off from a middle sectionof the column II and rectified in side columns 18 and 19. The residueaccumulating in the sump of column Il is forced by the pump 20 through'the tube furnace 21, the pump keeping said residue in constantcirculation. Here again a portion of the residue is evaporated, Whilst aportion corresponding to the inow to column II is forced by the pump 22through the tube furnace 23 to a middle section of column III.

From the head of this column the fraction. h, i. e.

' l'anthracene oil I, is taken off, which by Way of the heat Y.exchanger 3 and cooler 24 passes into receiver 25. This receiver isconnected to vacuum pump 26. As the coolers 14 and 24 are exposed torelatively high temperatures, salts are deposited from cooling water inthe coolers on the water side and thereby hinder heat exchange in thecooler. Therefore, it is advisable to use in these coolers a suitabletar oil as cooling medium. i

The pump 27 conveys a portion of fraction h into the head of the columnIII, the inflow being so vregulated that the temperature at the head ofthis column is in the range of to 160 C.

In order to obtain the highest possible yield of oil while using thelowest possible distillation temperature, the absolute pressure in thecolumn III is reduced to about 10-100, preferably 10-40 mm. Hg. In orderto assist this expedient, liquid is drawn o from an upper tray of columnIII and cooled in the heat exchanger 4. From there the pump 28 conveysthe cooled liquid back to the adjacent section of column III. Thiswithdrawal of heat reduces the amount of vapors in the upper part of thecolumn and on the way to the head after the heat exchanger, causingconsiderable reduction of the pressure loss at these places.

The fraction i. e. anthracene oil II, is conveyed from a middle sectionof column III into side column 29 and is rectified there. Thissubsidiary column is provided, like the side columns of the precedingmain columns with reheaters which can be heated with superheatedwatervapor. A particularly economical heating can be obtained, if accordingto the invention a portion' of the residues circulating through the tubefurnaces is branched off behind the tube surfaces and if this branchedotf material gives off its condensation heat and some of its sensibleheat to the respective reheater of a side column. The fractionation inthe side columns leads to a particularly sharp separation, if thetemperature in the sump of the side columns is kept unchanged bycorresponding regulation of the inow.

The portions of the charge, which do not evaporate on the supply tray ofcolumn III, are subjected in the lower part 30 of the column to aparticularly high vacuum produced by the vacuum pump 31. By suitableconstruction of the lower column part, the pressure loss betweenevaporation and condensation can be kept so small that an absolutepressure of about 10 mm. Hg can be maintained. The condensate thusobtained is discharged separately, as heavy oils j by the condenser 32while the pitch, which has a temperature of about 320 to 380 C. owsfirst into heat exchanger 5 and from there to the vacuum receivers.

Depending on the number of the desired fractions, the individual maincolumns can be provided with one or more side columns. Furthermore, thenumber of main columns can be increased beyond those shown in the aboveexample and the drawing. By connecting or disconnecting side columns,the number and nature of the fractions can be Widely varied, so that themethod of the invention can be easily adjusted to the market conditions.

It will be understood from the above that the process of the presentinvention results in the recovery of a considerable number of sharplycut fractions and products from coal tar, by the application of a highamount of heat in such manner that no undue thermal decomposition occursand, due to the re-use of heat, the consumption of heat in the processis kept at a very low level. These results are attained by carrying outcontinuous fractionation of coal tar in several steps while subjectingit to heating at temperatures which increase from step to step but atpressures which decrease from step to step up to a high vacuum in thelast step, in the above described novel manner, including the use ofmain columns, each of which is provided with at least one side columnand causing the distillation residue of each main column, except thelast column, to be reheated by circulation through a reheater and thento be returned to the column under expansion and partial evaporation,whereupon the thus reheated sump liquid, including the pitchingredients, is passed into a rectifying stripping zone of the adjacentmain column.

It will be also understood that the invention is not limited to thespecic steps, conditions, devices and other details described above andillustrated in the drawings and can be carried out with variousmodifications. For example, instead of the 3 main columns shown in thedrawings, more main columns, e. g. 4 or 6 main columns can be used andthe number of side columns can also be varied.

In carrying out the present invention branched of portions of thematerial treated in the main columns are introduced into an uppersection of a side column for further rectification, after which the sumpliquid of the side column is discharged, whilehead products of the sidecolumn are reintroduced into the respective main column. The maincolumns are conventlonal fractionating columns containing e. g. 40 to 60trays, and the Column Head Tem- Sump Tem- Head Pressure,

perature, C. perature, C. mm. Hg

. 80 to 140-.- abt. 280 to 320.. abt. 760 to 1,100. 110 to 180-- abt.150 to 220 abt. 800 to 1,260.

120 to 190.. abt. 300 to 350 abt. 40 to 400. 130 to 200.. abt. 140 to220" abt. 50 to 425. 140 to 210-- abt. 160 to 280-. abt. 60 to 450. 150to 260.- abt. 320 to 380-. abt. 10 to 100. 210 to 340.. abt. 240 to340.. abt. to 125. 240 to 380.- abt. 10 to 40.

The side columns may be provided with reboilers as shown in the drawing,or the sump products of the side columns may be reheated by circulationthrough tube furnaces, as the sump products of the main columns.

The lower part of the main column III is provided with evaporatingplates which will cause no substantial pressure loss. Thereby it ispossible to supply heat eventually also to the sump of the main columnlII, so that the major part of the volatile constituents of the pitchcan be distilled 01T.

The terms naphthalene oil I and naphthalene oil II are used to denotefractions containing mainly naphthalene as crystallizable solidhydrocarbon and having a boiling range of 215 to 221 C. and 237 to 241C., respectively. The term anthracene oil I and anthracene oil II, areused to denote fractions containing anthracene in mixture with othercrystallizable ingredients and having boiling ranges of 332 to 344. C.and 344 to 366 C., respectively.

Reference is made to` our co-pendng application Ser. No. 192,702, filedon October 28, 1950, for Continuous Distillation of Tars and HydrocarbonOils of All Kinds, now abandoned, of which this is acontinuation-impart.

What is claimed is:

1. A process for the continuous rectication of coal tar to sharply cutfractions and to enriched individual components of the coal tar, saidfractions and com ponents having, under a pressure of 40 mm. Hg, boilingpoints up to 350 C., which comprises a combination of the followingsteps: (a) introducing the coal tar for stepwise fractionation into aplurality of main fractionat- 6 ing steps having a head zone, a middlerectifying stripping zone and a sump zone and following each other inseries, whereby the crude coal tar is introduced into said middle zoneof the first main fractionating step, in which water and low boilingoilsare driven oif, and which is operated under an absolute pressure ofabout 760 to 1100 mm., at a head temperature of about to 140 C., and asump temperature of about 280 to 320 C., and the temperature isgradually increased and the pressure is gradually decreased in thesubsequent main lfractionating steps in the direction of the last mainfractionating step which is operated under an absolute pressure of 10 to100 mm. Hg, a head temperature of about 150 to 260 C. and a sumptemperature of about 320-3 80 C., and each main fractionating step hasbranched oi therefrom at least one side fractionating step for furtherfractionating parts of the tar branched olf from said main steps; (b)subjecting sump liquid accumulating from each main fractionation step,with the exception of the last main fractionating step, tonon-destructive indirect heating in a step consisting of continuousforced circulation through a tube furnace and continuous separation intovaporous and liquid ingredients upon its return to the sump zone of thesame main fractionating step under expansion and partial evaporation, inorder to mainly supply to the process the heat required for said sharpfractionation; (c) passing sump liquid from the sump zone `of each mainfractionation step, with the exception of the last but one mainfractionation step, into the rectifying stripping zone of the succeedingmain fractionating step and (d) continuously discharging the productsthus separated by fractionation.

2. A process as claimed in claim 1, which comprises a rst, second andlast main fractionating step, the second fractionating step beingoperated under an absolute pressure of 40-400 mm. Hg, at a headtemperature of about to 190 C. and a sump temperature of about 300 to350 C.

3. A process as claimed in claim 2, in which the residue of materialfrom the second main fractionating step is subjected to indirect heatingprior to its introduction into the last main fractionating step andsubjected to expansion and rectification under a pressure of 10 to 40mm. Hg for the rectiiication of the residual volatile tar components.

4. A process as claimed in claim 2, in which the sump product in thelast main fractionating step is subjected to a higher vacuum at thelower level of said last main fractionating step than the vacuum at anupper level of this main fractionating step and the vapors formed atsaid lower level are separately discharged from said last mainfractionating step.

References Cited in the le of this patent UNITED STATES PATENTS RadaschMar. 30, 1954 OTHER REFERENCESA Petroleum Renery Engineering, by Nelson,3rd edition, McGraw-Hill Book Co. (1949), pages 208-9 relied f On.

1. A PROCESS FOR THE CONTINUOUS RECTIFICATION OF COAL TAR TO SHARPLY CUTFRACTIONS AND TO ENRICHED INDIVIDUAL COMPONENTS OF THE COAL TAR, SAIDFRACTIONS AND COMPONENTS HAVING, UNDER A PRESSURE OF 40 MM. HG, BOILINGPOINTS UP TO 350* C., WHICH COMPRISES A COMBINATION OF THE FOLLOWINGSTEPS: (A) INTRODUCING THE COAL TAR FOR STEPWISE FRACTIONATION INTO APLURALITY OF MAIN FRACTIONATING STEPS HAVING A HEAD ZONE, A MIDDLERECTIFYING STRIPPING ZONE AND A SUMP ZONE AND FOLLOWING EACH OTHER INSERIES, WHEREBY THE CRUDE COAL TAR IS INTRODUCED INTO SAID MIDDLE ZONEOF THE FIRST MAIN FRACTIONATING STEP, IN WHICH WATER AND LOW BOILINGOILS ARE DRIVEN OFF, AND WHICH IS OPERATED UNDER AN ABSOLUTE PRESSURE OFABOUT 760 TO 1100 MM., AT A HEAD TEMPERATURE OF ABOUT 80 TO 140* C., ANDA SUMP TEMPERATURE OF ABOUT 280 TO 320* C., AND THE TEMPERATURE ISGRADUALLY INCREASED AND THE PRESSURE IS GRADUALLY DECREASED IN THESUBSEQUENT MAIN FRACTIONATING STEP WHICH THE DIRECTION OF THE LAST MAINFRACTIONATING STEP WHICH IS OPERATED UNDER AN ABSOLUTE PRESSURE OF 10 TO100 MM. HG, A HEAD TEMPERATURE OF ABOUT 150 TO 260* C. AND A SUMPTEMPERATURE OF ABOUT 320-380* C., AND EACH MAIN FRACTIONATING STEP HASBRANCHED OFF THEREFROM AT LEAST ONE SIDE FRACTIONATING STEP FOR FURTHERFRACTIONATING PARTS OF THE TAR BRANCHED OFF FROM SAID MAIN STEPS; (B)SUBJECTING SUMP LIQUID ACCUMULATING FROM EACH MAIN FRACTIONATION STEP,WITH THE EXCEPTION OF THE LAST MAIN FRACTIONATING STEP, TONON-DESTRUCTIVE INDIRECT HEATING IN A STEP CONSISTING OF CONTINUOUSFORCED CIRCULATION THROUGH A TUBE FURNACE AND CONTINUOUS SEPARATION INTOVAPOROUS AND LIQUID INGREDIENTS UPON ITS RETURN TO THE SUMP ZONE OF THESAME MAIN FRACTIONATING STEP UNDER EXPANSION AND PARTIAL EVAPORATION, INORDER TO MAINLY SUPPLY TO THE PROCESS THE HEAT REQUIRED FOR SAID SHARPFRACTIONATION; (C) PASSING SUMP LIQUID FROM THE SUMP ZONE OF EACH MAINFRACTIONATION STEP, WITH THE EXCEPTION OF THE LAST BUT ONE MAINFRACTIONATION STEP, INTO THE RECTIFYING STRIPPING ZONE OF THE SUCCEEDINGMAIN FRACTIONATING STEP AND (D) CONTINUOUSLY DISCHARGING THE PRODUCTSTHUS SEPARATED BY FRACTIONATION.